Mendel’s Experiments

MENDEL’S EXPERIMENTS ON THE GARDEN PEA

Sir Gregor Johann Mendel (1822 to 1884) was Austrian monk who used garden pea (Pisum sativum) for his experiments and published his results in 1865. His work, however, was rediscovered in 1900, long after Mendel’s death, by Tschermark.

The transmission of characters from one generation to the next, that is from parents to offspring is known as heredity.

Variation means differences between parents and offspring or between offspring of same parents or between members of the same population.

Mendel’s Experiments

Mendel designed his experiments such that a pure tall variety of pea plants could be crossed to a pure dwarf variety. The anthers from flowers of tall plants were removed and their stigmas dusted with pollen from flowers of dwarf plants. The reverse experiment was also carried out. In the following spring, seeds from the new plants were collected and sown. He found that all the plants of this generation called first filial generation or F1 grew to be tall plants. He allowed them to self pollinate. Again he collected the seeds. The following year, after the seeds had been sown, he found that three quarters of these plants were tall and the rest dwarf. He repeated the experiment several times and found that the ratio of tall to dwarf plants was 3: 1. In this way he tried to cross pea plants with seven such contrasting characters. These were

1. Red flowered and white flowered plants;
2. axillary flowered (flower arising in the axial of the leaf) and terminal flowered (flower arising at tip of stalk)’
3. yellow seeded versus green seeded;
4. round seeded verses wrinkled seeded
5. green pod versus yellow pod
6. plants with inflated pods and
7. pure tall plants versus pure dwarf plants. Plants with such contrasting characters exist in varieties that are ‘self pollinating’ so that generation after generation they express only on type of feature

Mendel’s Principles (laws) of Inheritance

Basing on the results of his experiments. Mendel postulated the following laws of heredity.

Crosses considering the inheritance of one feature only are called monohybrid crosses. Mendel also tried crosses involving two contrasting features such as tail and red flowered with dwarf and white flowered plant such a cross is termed dihybrid cross.

1. Law of segregation or purity of gametes. At formation of gametes, the two chromosomes of each pair separate (segregate) into two different cell which from the gametes. This is a universal law and always during gamete formation in all sexually reproducing organisms, the two factors of a pair pass into different gametes. Each gamete receives one member of a pair of factors and the gametes are pure.
2. Law of dominance. Inheritance of many features (e.g. eye colour, flower colour, seed shape etc.) is controlled by one pair of genes when the two genes are of the same kind (e.g. brown colour of eyes, red colour of flower) the condition is termed homozygous. When a pair of chromosomes has the gene controlling the same feature  (flower colour) in two different forms (red flower gene on one chromosome and white flower gene on its pair termed its homologue the condition is termed heterozygous. The second law of inheritance maintains that of the two contrasting genes of a pair, the expression of one in dominant over that of the other. The recessive feature (e.g. dwarfness of the plant) shows up only when its gene is in the hornozygous condition. The law of dominance was found to be true in both monohybrid and dihybrid crosses in cases of the characteristics studied by Mendel in the garden pea.
3. Law of independent assortment meaning whereby that in the inheritance of two features (each feature controlled by a pair of genes), genes for the two different features are passed down into the offspring independently i.e. the Segregation of one pair of factors is independent of the other.

Red tall white dward

parents TIRR x ttrr

gametes IR tr

F1 Tt Rr tall, red (self)

F2 progeny worked out below.

Dihybrid phenotypic ration

9 Tall red: 3 tall dwarf; 3 dwarf red; 1 dwarf white

Result showing independent assortment in two pairs of genes. R stands for red flower colour, r for white flower colour, T for tall gene and t for gene for dwarfness.

You would have noticed that the composition of genes or genotype controls the outside expression which we can be seeing , that is the phenotype. The ratio of progeny in the crosses is therefore , the phenotypic ratio. However, more and more scientists began to devise genetic experiments, it became clear that Mendel’s laws do not hold true in all cases. We shall learn about the deviations from Mendel’s laws such as incomplete dominance, co dominance etc.

Reasons for Mendel’s success

1. His choice of material was good. He selected garden pea which has g short life cycle, has self pollinated bisexual flowers with closed corolla so pollination

Genes in male and female gametes TR Tr tR

Heredity can be easily controlled. Also it is easy to cultivate pea plants and a large number of pure line plants with several pairs of contrasting characters were available.

2. His selection of traits : The seven pairs of contrasting characters of pea plant considered by Mendel in his experiments were responsible for the desired.

3. Mendel’s Technique : His technique of experimentation was excellent and was as follows (i) Homozygous pure lines plants with contrasting characters were crossed.

(ii) Self Pollination  was prevented by removing stamens.

(iii) Female plants were dusted with pollen grains from another plant with the opposite feature and were tied in a bag to prevent any further pollination.

(iv) Seeds were collected and shown in time.

(v) He considered the inheritance of one character at a time.

(vi) The results were analysed statistically.

(vii) He performed reciprocal crosses and test crosses to confirm the results.